The start-up behavior of many linear integrated circuits is determined by external component selection. This is particularly true for a shunt regulator that operates over a wide range of voltages and has various types of external components interfaced with it. In order to allow flexibility for the end-user, it is desirable to have the shunt regulator circuit operate over a large range of external factors, such as input voltage, and components such as input and output capacitors, etc.
FIG. 1 shows the fundamentals of a typical closed loop type linear shunt regulator 10. The regulator has a device 12 such as a transistor for shunting excess bias current to ground to maintain regulation. The transistor is of suitable capacity for the amount of current being regulated. Input voltage Vin is supplied through a resistor R1 to the regulating device 12 drain. The source of the device 12 is connected to ground. An output bypass capacitor C1 has one end connected to the junction of the device 12 drain and R1 and the other end connected to ground. The regulator output voltage Vout is taken at the upper end of C1, which is included to reduce noise and improve transient response. The resistor R1 sets a value of a current IBIAS that is determined by the formula:
  IBIAS  =                    Input        ⁢                                  ⁢        Voltage            -              Output        ⁢                                  ⁢        Voltage                    R      ⁢                          ⁢      1      
The regulator 10 is a closed loop type circuit that has an error amplifier 14 whose output is connected to the gate electrode of the regulating device 12 to control its conduction state. The negative (inverting) input to the error amplifier 14 is a highly stable reference voltage, Vref, from a bandgap core 16, which is the target value for the regulator output voltage Vout. Many conventional circuits are known for implementing the bandgap core 16. The bandgap core 16 is connected between a point of Vout and ground to receive an operating voltage. The error amplifier 14 positive (non-inverting) input is the voltage Vout which completes the closed loop. The regulator 10 operates so that if the voltage Vout exceeds the reference voltage Vref applied to the error amplifier 14, then the error amplifier 14 produces an output signal that causes device 12 to conduct current, which it dissipates as heat. This regulates the output voltage Vout to be at the target value.
When a regulator circuit such as that shown in FIG. 1 is turned on, it takes some time for the circuit components to stabilize so that it will be operating in a linear range of the error amplifier 14 in which it is able to regulate Vout. The start-up requirements of a typical shunt regulator such as that of FIG. 1 are principally determined by the values of the bypass capacitor C1 and the bias current setting resistor R1. If C1 ranges from 10 pF to 10 μF, and IBIAS ranges from 10 μA to 10 mA then the start-up time constant can vary by as much as nine orders of magnitude. The start up time constant is defined as the time required for the regulation loop to reach its linear operating range as set by the external factors and component values. In practical terms, this means that the shunt regulator must be able to respond in nanoseconds to prevent large amounts of Vout overshoot from the target value when C1 is small and IBIAS is large. Conversely, the regulator must avoid motor-boating and meta-stable states with long start-up time constants of seconds due to a large C1 value and a small IBIAS value.
Conventional shunt regulators allow the start-up to be controlled by the error amplifier steady-state regulation loop. The minimum start-up time constant to avoid Vout output overshoot is limited by the amount of time it takes for the regulation loop to become biased into its linear operating region and also the bandwidth of the error amplifier. This generally requires tens to hundreds of microseconds. Very large start-up time constants (seconds) increase the likelihood that the loop will motor-boat or get trapped in a metastable state because the loop is held out of its linear regulating region for long portions of the start-up time and its ability to control the output voltage varies.
The steady-state regulation loop is typically optimized and compensated for steady-state operation and not for start-up behavior. The steady state regulation loop requires certain bias conditions and a certain amount of time to behave in a predictable, well controlled manner. Either one or both of these factors are not met over certain ranges of start-up time constants.
Accordingly, a need exists to provide a shunt regulator that does not have the above-described disadvantages of the prior art circuits, even though the external factors and components of the circuit might vary over a large range.